The present invention relates to a turbocharger for use with automotive engines or the like, and more particularly to a turbocharger housing construction for a turbocharger having variable pitch inlet vanes.
Turbochargers for use with automotive engines include turbine and compressor wheels supported on and coupled by a shaft rotatably supported by bearings. Since clearances around the bearings are small and the heat of exhaust gases is transmitted from the turbine housing to the bearings, a large amount of lubricating oil is supplied to the bearings to lubricate and cool the bearings. When the engine is shut off, the supply of the lubricating oil is also stopped. Therefore, in the event of an engine shutdown during high-speed operation of the turbocharger, an unwanted phenomenon known as heat soak back is caused to burn and carbonize lubricating oil remaining around the bearings and in oil passages. The carbonized lubricating oil deposit will reduce the durability of the turbocharger.
To provide against the heat soak back phenomenon, there has been proposed a turbocharger having a water jacket in the vicinity of shaft bearings (see, for example, Japanese Laid-Open Utility Model Publications Nos. 58-124602, 61-35707, and 61-37791). In the proposed turbocharger, the heat remaining around the shaft bearings is removed by heat of vaporization of cooling water in the water jacket for thereby preventing remaining lubricating oil from being burned and carbonized at the time of heat soak back. As a result, the durability of the turbocharger is increased.
However, since oil supply and drain passages of relatively large cross-sectional areas for supplying and discharging lubricating oil are defined near the shaft bearings, the volume of the water jacket is small to avoid physical interference between the water jacket and the oil supply and drain passages. Under severe operating conditions, therefore, the shaft bearings may not be satisfactorily cooled by the limited amount of cooling water in the water jacket. In a recently proposed turbocharger with variable pitch inlet vanes, particularly, a mechanism for adjusting variable restrictions defined between fixed and movable vanes is positioned in the neighborhood of the shaft bearings, and hence a large space for the water jacket cannot be provided near the shaft bearings.
As disclosed in Japanese Patent Publication No. 38-7653, a known turbocharger with variable pitch inlet vanes includes an annular array of movable vanes disposed in a throat around a turbine wheel to provide variable restrictions for passage of exhaust gases therethrough. When an engine associated with the turbocharger operates in a low-speed range, the movable vanes are actuated to reduce the opening of the variable restrictions. Because the variable restrictions are defined between the movable vanes, however, the opening of the variable restrictions is greatly affected by even a small change in the angle of inclination of the movable vanes. As a result, the opening of the variable restrictions cannot accurately be controlled when the opening is relatively small.
There has been proposed a turbocharger capable of accurately controlling the opening of variable restrictions even when the opening is small, as disclosed in Japanese patent application Ser. No. 61-124996 filed May 30, 1986 by the present applicant. In the disclosed turbocharger, a turbine wheel is surrounded by a turbine housing including a top plate and a back plate, and fixed vanes are secured to the top plate and movable vanes are mounted on pins supported by the back plate. The fixed and movable vanes are disposed outside of and adjacent to a throat around the turbine wheel to provide variable restrictions for passage of exhaust gases.
The fixed vanes are attached to the top plate, and the movable vanes are supported on the back plate which is separate from the top plate. Consequently, it is difficult to accurately establish a gap or clearance between the ends of the movable vanes which are mounted on the pins and the fixed vanes due to an allowed assembling tolerance. With an improper clearance setting, the movable vanes may suffer malfunctioning, or the turbine efficiency may be lowered. The clearance should preferably be small in order to prevent an exhaust leakage for higher turbine efficiency. If the clearance were too small, however, the movable vanes would interfere with the fixed vanes when the top plate is heated, and the movable vanes would not smoothly be operated.
According to a turbocharger disclosed in Japanese Patent Publication No. 61-37791, compressor and turbine housings are joined by a central housing, and compressor and turbine wheels housed in the compressor and turbine housings are coupled by a shaft rotatably supported in the central housing. Inasmuch as the turbocharger operates at high temperature under the heat of exhaust gases, the housings are made of a heat-resistant material, and the central housing is cooled, to prevent seizure of the shaft. The turbine housing and the central housing are held in direct contact with each other through a relatively large area. Thus, the amount of heat transmitted from the turbine housing to the central housing is large. Since a relatively large tolerance is permitted when assembling the central and turbine housings together, the clearance between the turbine housing and the turbine wheel cannot accurately be controlled.
In the turbocharger disclosed in Japanese patent application Ser. No. 61-124996 referred to above, a base plate is fitted in the turbine housing and between the turbine and central housings, and the top plate is fixed to the base plate in the turbine housing in surrounding relation to the turbine wheel, which can be driven by exhaust gases applied thereto. Heat transfer to the central housing is prevented by the base plate. The top plate is disposed concentrically around the turbine wheel to define a clearance (nozzle) around the top plate and between the top plate and the turbine wheel and to accurately control the clearance.
The base plate fitted in the turbine housing has its outer peripheral surface held in intimate contact with an inner peripheral surface of the turbine housing. When the turbine housing is subjected to thermal strain due to the heat of exhaust gases, the base plate also suffers thermal strain, thus bringing the turbine wheel and the top plate out of concentricity. More specifically, the turbine housing is asymmetrically shaped because of a scroll passage defined therein for producing a swirl in the exhaust gases and an exhaust inlet opening tangentially into the scroll passage. The turbine housing therefore undergoes large localized thermal strain, and the top plate is brought largely out of concentricity due to its thermal strain. As a consequence, the turbine wheel and the top plate may interfere with each other, and the amount of exhaust gases leaking around the turbine wheel is increased thereby to lower the turbine efficiency.
Some turbochargers include an annular shroud disposed in a turbine housing which accommodates a turbine wheel. The turbine housing includes an exhaust passage for applying exhaust gases to the turbine wheel, the exhaust passage having an exhaust nozzle for speeding up the exhaust gases.
With a turbocharger having variable pitch inlet vanes, variable restrictions are defined by movable vanes and positioned in series with or independently of the exhaust nozzle. The movable vanes are tiltably disposed in the exhaust passage and slidably held against the shroud.
During operation of the turbocharger, the shroud is heated and deformed by the heat of exhaust gases, and the clearance of the exhaust passage, particularly the exhaust nozzle, is varied. The shroud which has thus suffered thermal strain is apt to interfere with the movable vanes, which may not be operated smoothly.
When the shroud is cooled and shrunk, a gap is created between the inner peripheral edge of the shroud and the central housing, allowing exhaust gases to leak through the gap.